1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus.
2. Description of the Related Art
An image forming apparatus of an electrostatic photographic system generally forms an image by executing steps of charging, exposure, development, transfer, cleaning, charge erasure, and fixing. In the forming step, for example, a photoreceptor drum driven to rotate has its surface uniformly charged by a charging device and the surface of the photoreceptor is irradiated with laser light by an exposure device so that an electrostatic latent image is formed on the surface of the photoreceptor charged. Subsequently, latent image formed on the surface of the photoreceptor is developed by a developing device and a toner image is formed on the surface of the photoreceptor. And then, by a transferring device, the toner image on the photoreceptor is transferred onto a member to be transferred. After that, due to the pressure and heating by a fixing section, the toner image transferred onto the to-be-transferred member is fixed thereto. Further, in a cleaning device, a residual toner remaining on the surface of the photoreceptor is removed and collected in a predetermined collecting section. Moreover, in an electricity removing device, residual charges on the surface of the photoreceptor after cleaning are removed, thereby preparing for next image formation.
As a developer for developing the electrostatic latent image formed on a surface of the photoreceptor, a one-component developer formed of a toner only, and a two-component developer composed of a toner and a carrier are commonly used. Although the one-component developer is advantageous in that the developing section can be of a simple structure with no need of an agitating mechanism or the like for mixing the toner and the carrier evenly since the one-component developer contains no carrier, the one-component developer has disadvantages such that stabilization of charge amount of the toner is not easy. Although the two-component developer is disadvantageous in that the developing section has a complicated structure with a need for an agitating mechanism or the like for mixing the toner and the carrier evenly, the two-component developer is often used for a high-speed image forming apparatus and a color image forming apparatus since the toner is stably charged.
As a developing device in which a two-component developer is generally used is a developing device including a developing tank, a developing roller, an agitating member, a regulating member, a chute, and the like. The developing tank supports the developing roller and the agitating member so as to rotate freely and contains a developer therein. The developing roller rotates with a developer layer borne on the surface thereof, and supplies toner to an electrostatic latent image on the surface of a photoreceptor to form a toner image. The agitating member uniformly agitates the developer in the developing tank to feed toward the developing roller. The regulating member regulates layer thickness of the developer layer on the surface of the developing roller.
In the two-component developer, in order to correspond to increase in printing speed, colorization, and further energy saving, a particle diameter and a softening point of toner are being reduced, and such toner has a drawback of being aggregated by heat easily. Accordingly, when a temperature in the developing device increases due to frictional heat in agitation in the developing device, a temperature of the developer increases to cause a problem that a toner aggregate is generated and fluidity of the developer is deteriorated to generate unevenness of an image density. In particular, there is a problem that no air flow is generated inside the developing roller and heat is likely to be filled therein so that a temperature gradually increases due to long-time use.
Against this problem, for example, in a developing device described in Japanese Unexamined Patent Publication JP-A 2002-229330, a passage for communication in an axial direction is provided in a developing roller, into which air is sent by a blowing section, and the sent air is guided to an inner circumferential surface of the developing roller by a spiral member to thereby enhance cooling efficiency of the developing roller.
However, since a developing roller for two-component developer has a plurality of magnets (magnet rollers) inside the developing roller in order to form a magnetic brush, it is difficult to secure a space for providing a spiral member in a passage, like the developing device of JP-A 2002-229330. In particular, in a miniaturized developing device, since a developing roller whose diameter is small is used, it is difficult to provide a spiral member with a sufficient width, thus cooling efficiency is not enhanced. As a result, it is impossible to sufficiently prevent deterioration in image quality such as image unevenness clue to increase in a temperature of the developer.